Effect of Reynolds number on impact force and collision process of a low-velocity droplet colliding with a wall carrying an equal-mass deposited droplet

The impact force and collision process of a glycerol or a glycerol aqueous solution droplet colliding with a horizontal wall carrying an equal-mass deposited droplet are presented. A piezoelectric sensor-based force measurement platform, a high-speed camera and the volume-of-fluid numerical simulation method were used to measure the impact force and to explore the underlying mechanism. Results indicate the collision can be categorized into two regimes in terms of Reynolds number, i.e. a viscous regime and an inertial regime. For the collision in the viscous regime, two high-pressure regions are formed first at the edge and center of the deposited droplet. Then the two high pressures will drop slowly. This results in a single-peak force–time curve. For the collision in the inertial regime, an annular jet is formed first between the two droplets, with a high-pressure region generated at the center of the deposited droplet. The falling of the annular jet on the wall leads to a pressure rise at the edge of the deposited droplet. The edge pressure then increases further, resulting in a second force peak. The annular jet is key for the occurrence of double-peak force–time curve and a sign of the inertial regime. The Reynolds number of the regime boundary is about 101 to 107. The single dimensionless impact peak force decreases gradually with Reynolds number in the viscous regime. The first dimensionless impact peak force changes slightly in the inertial regime.